The present invention is related generally to telecommunication and, more specifically, to a technique for detecting and compensating for time of arrival errors in a telecommunications system.
Emergency services are often requested using telephone numbers, such as xe2x80x9c911.xe2x80x9d If the caller is in a fixed location, such as a residence, computer systems track the telephone number of an incoming telephone call using automatic number identification (ANI) and quickly determine the address from which the call originated. Thus, it is a relatively simple task to determine the location from which emergency services are requested.
The location of a user requesting emergency service requests via mobile communications, such as cellular telephones, personal communication systems (PCS) devices and the like, is not as easily determined. Radio triangulation techniques have long been used to determine the location of a mobile unit. However, such radio triangulation techniques are known to be inherently inaccurate. Errors on the order of thousands of meters are not uncommon. However, such errors are unacceptable for the delivery of emergency services.
The Federal Communications Commission (FCC) has ordered changes in communication technology that will permit greater accuracy in location determination. In the case of mobile communications, the FCC has generated a rule that requires infrastructure based location systems to have an accuracy of 150 meters 67% of the time (and an accuracy of 300 meters 95% of the time). For systems that require modified handsets, the FCC has decreed that such systems must determine location within 50 meters 67% of the time (and 150 meters 95% of the time).
Radio location systems use time of arrival (TOA) signals coming from different transmitters of known positions to triangulate and estimate the mobile unit location. However, time of arrival signals are often distorted or erroneous due to multiple transmission paths. FIG. 1 illustrates an example of multiple transmission paths that may be experienced by a mobile phone in a vehicle 10. In the example illustrated in FIG. 1, the mobile unit 10 is receiving signals from transmitters 12 and 14 mounted atop towers. In the example of FIG. 1, the mobile unit 10 receives a signal directly from the transmitters 12 and 14, but also receives signals from the transmitter 14 that have reflected off nearby buildings. Thus, the mobile unit 10 receives a number of signals from the transmitter 14. In the example illustrated in FIG. 1, the mobile unit 10 is not within the line of sight (LOS) of the transmitter 16. That is, buildings or other structures block the direct line of sight between the mobile unit 10 and the transmitter 16. However, the mobile unit 10 still detects signals from the transmitter 16 that are reflected off buildings or other structures or are defracted around edges of buildings or other structures. In addition, the mobile unit 10 receives signals from a transmitter 16 mounted atop a building and may also receive signals from a global positioning system (GPS) satellite 18 in orbit about the earth. As a result, the mobile unit 10 receives multiple signals from the transmitter 16, none of which are direct LOS signals. Signals from the GPS satellite 18 may also comprise LOS signals and reflected signals. As a result of such multipath signals, the time of arrival measurements by the mobile unit are subject to error. Such errors can be significant in the presence of multipath signals, thus making it difficult or impossible to achieve the FCC directives with regard to location accuracy. Therefore, it can be appreciated that there is a significant need for a system and method to improve TOA measurements for mobile location systems. The present invention provides this and other advantages that will be apparent from the following detailed description and accompanying figures.
The present invention is embodied in a system and method for correction of multipath errors in a telecommunication device location system. In one embodiment, the system comprises a receiver that receives data transmitted from a remote transmitter located at an unknown distance from the receiver. An analyzer analyzes the date associated with the received data and generates location data related to the location of the receiver. The analyzer also calculates a correction factor based on a measured signal criteria to generate corrected location data.
In one embodiment, the receiver generates a correlation pulse when the received data is correlated to stored data. In this embodiment, the signal criterion is the pulse width of the correlation pulse. The correlation pulse may be modeled as a quadratic equation having a plurality of coefficients that are determined by amplitude values of the correlation pulse at predetermined times. In another embodiment, the receiver generates a signal strength indicator. In this embodiment, the signal criterion is the signal strength indicator.
The system may further comprise a position determining entity to determine the location of the receiver based on the corrected location data and a known location of the remote transmitter. The location data may be based on the time of arrival of the data received by the receiver. The time of arrival data may be calculated as a delay time or distance and the correction factor may be calculated as a correction time or correction distance.
In one embodiment, the receiver is a portion of the cellular telephone operating in an 800 MHz band and the analyzer calculates the location data based on a time of arrival of data transmitted from the remote transmitter in the 800 MHz band. Alternatively, the receiver may be a portion of a personal communication system operating in a 1900 MHz band and the analyzer calculates the location data based on time of arrival of data transmitted from the remote transmitter in the 1900 MHz band.
In yet another alternative embodiment, the remote transmitter is a global positioning system (GPS) satellite and the receiver receives the data signals from the GPS satellite. In this embodiment, the analyzer calculates the location data based on the time of arrival of data transmitted from the GPS satellite.
The system may further include a data structure to store data relating a selected signal criteria to one or more correction factors, wherein the analyzer provides a measure of the selected data as an input to the data structure and retrieves a correction factor stored in association with the measure of the selected criteria. The system may alternatively include a data structure to store mathematical function relating the selected signal criteria to one or more correction factors, wherein the analyzer calculates the correction factor using the selected criteria in the mathematical function.